Mr. A. Tulk on the Anatomy 0/ Phalangium Opilio. 155 



with each other, so as to constitute a band which passes (at least 

 upon the inferior surface of the abdomen,) from the inflected 

 margin of one segment to the distal border of the next, each fibre 

 having a distinct and separate attachment by its two extremities 

 to the elevations, above noticed, upon the internal surface of the 

 epidermis, while the intermediate portion lies free over the space 

 of two segments, and may be readily raised therefrom upon the 

 point of a fine needle. Examined with a power of 200 linear, 

 each of these fasciculi is seen to be made up of an aggregation of 

 smaller fibrillse, which are apparent from the ends of the former 

 when detached, giving to them a tufted or brush-shaped figure. 

 There are no traces of transverse strise upon these fibres, and they 

 do not dissolve or lose their form by maceration, like ordinary 

 muscular tissue. In certain situations, as upon the sides of the 

 abdomen and thorax, the form of their bands is triangular, the 

 apices being directed upwards, and I have observed them to pre- 

 sent there a very glistening nacreous lustre. 



A comparison of the above structure of the integument in Pha- 

 langium with that of a large species of Epeira {E. maculata, Fab.) 

 presents many obvious points of resemblance ; the chief difference 

 being, that in the latter the fibres of the corium are more closely 

 arranged, are smaller in proportion to the size of the animal, and 

 all take a transverse direction within the abdomen, and are more- 

 over enveloped both above and below by a considerable layer of 

 fat, which, with the exception of a few scattered adipose granules 

 in the same situation, can be scarcely said to exist in the harvest 

 spider. Notwithstanding the entire absence of transverse strise, 

 I am disposed to regard the fibres of the corium as constituting 

 a thin muscular expansion, which in the Arachnida generally may 

 take the place, and supply the function of, the regular longi- 

 tudinal muscles of the thorax and abdomen in insects, which 

 would otherwise be wanting in the family and class under consi- 

 deration. 



The body of Phalangium (PI. III. fig. I.) appears at first to be 

 composed only of a single piece, so intimately are the cephalo- 

 thorax and abdomen blended together ; but when examined 

 more closely, a distinction into these two principal divisions is 

 readily observed, though they are not so clearly indicated as in 

 some of the Tracheary Arachnida, such as Galeodes, where an in- 

 tervening constriction denotes their respective limits. I shall 

 consider, therefore, each of these portions of the dermo-skeleton 

 separately, commencing with the cephalo-thorax and the structures 

 appended thereto. 



The cephalo-thorax in P. Opilio (PI. III. fig. \. ct) occupies 

 about one-fifth of the entire length of the body. The upper plate 

 or buckler J darker in colour than the rest, is, as regards its con- 



M2 



